Mike,
I am sorry to report that you fully (and correctly) understand the situation here in Perth regarding pens / moorings. As I understand it, there is nothing more you can do than what you have done already.
I can add (IMHO) that you almost certainly will not get approval to lay a new mooring in the river. Many old worthless boats have been purchased simply to get the rights to their pen/mooring. Others have rented houses in Mandurah (complete with private jetty) just for the jetty aspects. I believe (but don't know for sure) that rents in Mandurah are falling (although from a very high base).
Best bet is to buy a boat with the pen/mooring rights or failing that, find a pen/ mooring first, then a boat.
If you think it this bad now, think how much worse it would have if we were still in the mining boom 2005 / 2007

.
PM me if you have any more questions.